Hey, if you have a relatively new computer you could go for a midi controller (evolution mk at $100) and roland virtual sound canvas (you could score this for under $50 if you look hard. Check out PG musics website for a special deal on this software)
This will give you everything you specified except for the 61 keys, the evolution only has 49 but has pitch shift and a modulator plus a numeric display, volume, etc. You get the following from sound canvas:
"The Roland Virtual Sound Canvas software will turn your Pentium or 6X86 based PC and 16 bit sound card into a complete musical instrument. With the Roland Virtual Sound Canvas software you will be able to play up to 16 instruments simultaneously from a palette of 354 different sounds. The VSC550W software also gives you 9 different drum kits as well as sound effects and digital signal processing like reverb, chorus and delay"
You can edit these sounds also (I think the editor might cost an extra $30 though) The latency is low enough that it isn't annoying. You will be much better of with this than a cheap yamaha or casio. You can easily use it with any midi sequencer like cakewalk, logic, etc. (your sound card may already have an interface) Plus you can use your midi controller with other programs like fruity loops or with sound modules if you decide to expand your gear inventory. A con is it's not portable (unless you are willing to lug your computer with you.)
just my 2 cents.